The invention relates to the field of solar cells, and in particular to thin crystalline silicon solar cells.
Thin crystalline Si solar cells are attractive because they use small volumes of Si material, and they should prove to be cost effective. However, the short optical path length in crystalline Si solar cells reduces the conversion efficiency of photons to carriers. FIG. 1 shows the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells (shown by line 102) and the photon number spectrum calculated from the air mass two (AM2) sun spectrum (shown by line 100). As shown, an IQE reduction starts at 0.8 xcexcm and goes to zero at xcx9c1.1 xcexcm, despite the fact that photon wavelengths up to 1.2 xcexcm can yield carrier generation (optical bandgap defined by absorption coefficient xcex1xcx9c10xe2x88x921 cmxe2x88x921 at 1.2 xcexcm).
The photon number spectrum 100 consists of various peaks that survive absorption and scattering in the air. Since the Si optical edge is at 1.2 xcexcm, photons in roughly half of the second peak 104 as well as the third peak 106 are wasted in thin Si solar cells because of the IQE reduction. This results in a low efficiency for current thin Si solar cells (the best reported one is xcx9c15%, as described in R. Brendel, xe2x80x9cCrystalline Thin-film Silicon Solar Cells from Layer-transfer Processes: a Review,xe2x80x9d Proc. 10th Workshop on Crystalline Silicon Solar Cell Materials and Processes, ed. by B. L Sopori, 117, 2000.). The efficiency of thin Si solar cells should at least equal the efficiency of bulk Si solar cells, which is 25%.
To overcome this deficiency in thin Si solar cells, light is typically bounced between the top and bottom surfaces of the solar cell. The current structure to perform this light trapping is the Lambertian top surface and Al backside electrode. The Al electrode has a reflectivity as low as 98%. Assuming that the Lambertian structure has the same internal reflectivity as Al, more than 99% of incident photons escape from cells if they bounce only 100 times between the surfaces. Yet, light with wavelengths near Si""s bandgap must be bounced back and forth more than 1000 times to be fully absorbed in current thin Si cells. This is because optical paths 10 cm or longer are required for 1.2 xcexcm light to be absorbed in Si and generate electron-hole pairs, while current thin Si solar cells are only about 50 xcexcm thick. Thus, it is very difficult to increase the absorption at near band edge wavelengths (near 1.2 xcexcm) by using the structure based on the Lambertian surface and Al reflectors.
The present invention alleviates problems with current light trapping in solar cells by using a photonic crystal as a backside reflector.
Thus, one aspect of the present invention provides for a solar cell that comprises a photoactive region; a Lambertian surface on the topside of the photoactive region; and a photonic crystal on the backside of the photoactive region.
Another aspect of the present invention provides a method of forming a solar cell that comprises forming a Lambertian surface on a topside of a photoactive region; and forming a photonic crystal on a backside of the photoactive region.